Effect of cattle grazing in the herbaceous vegetation of tropical dry forests in Morelos, Mexico

Background/Question/Methods: Ecosystems worldwide are intensely modified due to agricultural activities. Cattle grazing in natural communities modify floristic composition, structure, richness and biomass and affect its regeneration processes. We assessed the effects of cattle grazing on the herbaceous vegetation of a tropical dry forest at the Biosphere Reserve of Sierra de Huautla, Morelos, Mexico. Cattle is brought from other communities during the months of May-October to feed inside the secondary forest, no pastures are created. We took 12 random samples (50 x 50 cm) of herbaceous vegetation in ten 50 x 50 m plots during August-September 2008. We collected four plots of secondary forest excluded from livestock since November 2005, three plots of secondary forest without exclusions and three plots of conserved forest. Samples were separated by morphospecies and dried at 60 °C for 72 hr and weighed.

Results/Conclusions: We found 103 morphospecies, 14% were only found in the excluded plots (15 sp), 10.6% at non-excluded plots (11 sp) and 17.5 % were only found in conserved forest (17 sp). Fifty eight percent of the species were present in all habitats. The families with higher number of species were Poaceae, Asteraceae and Fabaceae. The species with higher biomass in all plots were grasses Lasiasis sp, Paspalum virgatum and Paspalum convexum. The plots excluded from cattle showed significantly higher species density (48.41 ± 7.44 sp/m²) compared to non-exclude plots (34.22 ± 2.31 sp/m²) and conserved forest plots (21.47 ± 8.84 sp/m²; F _(2,9)= 13.22, P < 0.005). After only two years without cattle excluded plots showed a significantly higher biomass (261.63 ± 41.20 g /m²) than the conserved (171.58 ± 60.29 g/m²) and non-excluded  (57.28 ± 15.13 g/m²; F _(2,9) = 19.54, P < 0.001). Shannon diversity index was higher in excluded secondary forest (2.80 ± 0.13) and non-excluded (2.65 ± 0.34) compared with conserved forest (1.73 ± 0.24). A low similarity was found between excluded plots and conserved forest (Jaccard index=0.23), perhaps due to the dispersal of exotic species by cattle. Furthermore, the closed canopy in the conserved forest does not have adequate microenvironmental conditions for the establishment of most herbaceous species. We conclude that cattle affect not only the biomass but it also changes the composition of herbaceous vegetation in the tropical dry forest.